In recent years, various information processing devices have been proposed following the advancement of information society and further, information recording devices such as hard disk drives (HDDs) for use in those information processing devices has been proposed. In such information recording devices, larger information recording capacities and higher information recording densities have been required for the purpose of miniaturization and higher performances of the information processing devices.
In a hard disk drive (HDD), in order to increase the information recording density, it is necessary to reduce a so-called spacing loss so that it is required to reduce a flying height (glide height) of a magnetic head that performs recording/reproduction with respect to a magnetic disk serving as a recording medium.
Since the magnetic disk is rotated at high speed during recording/reproduction, if the flying height of the magnetic head is reduced, there arises a greater possibility that the magnetic head contacts the surface of the magnetic disk so as to be crashed. In order to prevent such a crash of the magnetic head, it is necessary to finish the surface of the magnetic disk as an extremely smooth surface.
In order to realize such smoothness of the surface of the magnetic disk, a glass substrate has been used as a disk substrate in place of an aluminum substrate which has conventionally been widely used. This is because the glass substrate is excellent in surface flatness and substrate strength as compared with the aluminum substrate. As such a glass substrate, use is made of, in order to increase the substrate strength, a chemically strengthened glass substrate or a crystallized glass substrate increased in substrate strength by crystallization.
However, even if the smoothness of the surface of the disk substrate is ensured, it is further necessary to keep the surface of the disk substrate as a highly cleaned surface with no foreign matter. This is because if foreign matter is adhering to the surface of the disk substrate, it becomes a cause for a film defect of a magnetic thin film to be formed on the surface of the glass substrate or it becomes a cause for occurrence of a convex portion on the surface of the magnetic thin film so that a proper flying height of the magnetic head cannot be obtained.
On the other hand, as a magnetic head, use has been widely made of, in order to improve a signal strength upon record reproduction, a magnetoresistive head (MR head) or a giant magnetoresistive head (GMR head) using a magnetoresistive effect element (MR element), in place of a thin film head which has conventionally been widely used.
In such a magnetoresistive head using the magnetoresistive effect element, when a small roughness is formed on the surface of a magnetic disk, a thermal asperity trouble may occur to cause malfunction in reproduction or the reproduction may be impossible. This thermal asperity trouble is caused such that a convex portion formed on the surface of a magnetic disk due to foreign matter on a glass substrate generates adiabatic compression and adiabatic expansion of air in the vicinity of a magnetoresistive head due to high-speed rotation of the magnetic disk so that the magnetoresistive head generates heat to cause a change in resistance value of a magnetoresistive effect element and, therefore, electromagnetic conversion is adversely affected. That is, such a thermal asperity trouble can be generated even in the case where the magnetic head does not contact the magnetic disk.
Therefore, in order to also prevent the thermal asperity trouble, it is necessary to finish the surface of the magnetic disk as a surface that is extremely smooth and highly cleaned with no foreign matter.
As a cause for adhesion of foreign matter to the surface of the glass substrate, not only the surface shape of the magnetic disk but also the surface shape of an end face of the disk substrate are considered. Specifically, when the surface shape of the end face of the disk substrate is not smooth, this end face abrades a wall surface of a resin case so that dust (particles) of resin or glass is generated due to the abrasion. Then, such dust and dust in an atmosphere are captured and accumulated on the end face of the disk substrate. It is presumed that the dust accumulated on the end face of the disk substrate becomes a dust generation source and causes the adhesion of the foreign matter to the surface of the disk substrate in a later process or after mounted in a hard disk drive. Particularly, it is considered that the end face on the inner peripheral side of the glass substrate has a rougher surface shape as compared with the end face on the outer peripheral side and therefore is liable to capture the dust to thereby prevent the high cleaning of the surface of the glass substrate.
For the purpose of suppressing such a defect caused by the surface shape of the end face of the disk substrate, the present applicant has previously proposed a method of polishing an end face of a disk-shaped disk substrate by contacting a rotating polishing brush or polishing pad with the end face of the disk substrate, as described in Patent Document 1.
On the other hand, as described in Patent Document 2, proposal has been made of a technique of achieving improvement in substrate strength by removing, through chemical etching, a crack generated on an end face of a disk substrate made of glass. In this technique, although it is possible to prevent degradation of a strength of the glass substrate caused by the crack, the crack is broadened to form a hollow so that dust is easily captured instead and, therefore, the high cleaning of the surface of the disk substrate cannot be achieved.
Patent Document 1: JP 2000-185927 A
Patent Document 2: JP 7-230621 A